SauceLabs provide a great hosted Selenium service for cross-browser testing in the cloud, which allows you to run a Selenium test suite against multiple browsers using the SauceLabs API and SauceConnect.

They also provide integration with PHPUnit, so that you can plug your SauceLabs Selenium tests directly into your PHPUnit test suite.

Sometimes, though, you want to run the same Selenium test suite against a single browser on your local machine, before you run the full thing against all the browsers on SauceLabs.